The present invention relates to a process for preparing an improved precursor pitch for the production of carbon fibers of high quality.
There has been known a method of producing carbon fibers by melt-spinning a carbonaceous pitch, then rendering the resultant pitch fiber infusible and subjecting it to carbonization and, if required, to subsequent graphitization. In this connection, attempts have been made recently for improving the performance of carbon fibers as final product by subjecting such carbonaceous pitch to a physical or chemical treatment to thereby produce a pitch (hereinafter referred to as "precursor pitch") suitable for melt spinning.
As a method of producing the precursor pitch, there has been reported (U.S. Pat. Nos. 3,974,264, 3,995,014, 4,026,788, 4,032,430), for example, a method in which a carbonaceous pitch is heat-treated for a long time at a high temperature of around 400.degree. C. under reduced pressure or while introducing an inert gas. The precursor pitch obtained by this method is a mesophase pitch containing 40-90 wt.% of a pyridine- or quinoline-insoluble mesophase. But this method is disadvantageous in point of economy because the manufacturing cost is high; besides, high molecular weight components such as pyridine-insoluble or quinoline-insoluble components are produced in large amounts because the heat treatment must be conducted for a long time at a high temperature. If such high molecular weight components are present in large amounts in the precursor pitch, not only it becomes difficult to perform a continuous spinning stably in the subsequent melt spinning step, but also the resultant carbon fiber will be badly affected in its physical properties.
Further, the precursor pitch obtained by this method has an extremely high softening point because it contains a large amount of a high molecular weight component. As the softening point of the precursor pitch becomes higher, the melt spinning temperature also becomes higher, thus resulting in that the high molecular weight component is further increased in molecular weight by polycondensation, and at the same time there arises the problem that the pitch undergoes a thermal decomposition, generating a light gas and making spinning virtually impossible.
There has also been reported a method in which light components contained in a carbonaceous pitch are removed in advance by solvent extraction or vacuum distillation and thereafter the carbonaceous pitch is heat-treated. But this method is not only disadvantageous in point of economy because it requires an additional step for removing such light components in advance, but also it is inferior in point of physical properties of carbon fibers as final product, that is, only carbon fibers of inferior physical properties have heretofore been obtained by such method.
All of the precursor pitches prepared according to those conventional methods are still unsatisfactory for the production of carbon fibers of high performance, and involve problems also in point of economy.